1. Field of the Invention
The present invention relates to methods of forming a semiconductor device, and more particularly to a method for patterning a photoresist.
2. Background of the Related Art
As semiconductor devices have become highly-integrated, lithographic techniques capable of achieving a critical dimension of less than 0.25 .mu.m have become necessary. In order to improve a resolution of such a thin pattern, it is necessary to improve both the mask and the photoresist.
After a phase shift mask was first developed by Levenson in 1982, various studies for improving the phase shift mask have been carried out. Unfortunately, the Levenson-type or attenuation-type phase shift mask has no defect detecting equipment. Thus, it is difficult to use it for an actual semiconductor manufacturing process.
Recently, a half-tone phase shift mask has been widely used for semiconductor manufacturing processes. Because defect detecting equipment is available for a half-tone phase shift mask, it is possible to use it in an actual manufacturing process.
There have also been a number of studies about lithographic processes using a deep-UV exposure light having a wavelength of 200 to 300 nm. A new photoresist corresponding to the deep-UV exposure light was required, and IBM first developed a chemical amplification photoresist. The term "photoresist" refers to high polymer resins which form patterns by using the difference between dissolution of an exposed portion and of a non-exposed portion. The difference in dissolution properties are formed by a chemical reaction due to light exposure.
A method of forming a pattern using a background art half-tone phase shift mask and a chemical amplification photoresist will be now described with reference to FIGS. 1-6.
FIG. 1 is a vertical cross-sectional view of a background art half-tone phase shift mask. As shown therein, a substrate 1 is formed of an optically transparent material, and a semipermeable film 2 formed of chrome is formed on the substrate 1. A phase shift film 3 formed of silicon dioxide is formed on the semipermeable film 2. The semipermeable film 2 allows approximately 5 to 10% of incident light to pass there through. A portion in which the semipermeable film 2 is covered by the phase shift film 3 is a phase shift region or non-exposure region 4. A portion in which the substrate 1 is externally exposed is a light transmitting or exposure region 5.
FIG. 2 illustrates the phase and amplitude of light transmitted through the phase shift mask shown in FIG. 1. This is the light that would reach a chemical amplification photoresist formed on a substrate. Specifically, the light corresponding to the phase shift region 4 has a negative phase and a relatively low amplitude. The light corresponding to the light transmitting region 5 has a positive phase and a relatively large amplitude.
FIG. 3 illustrates only the intensity of the light transmitted through the phase shift mask in FIG. 1. The light corresponding to the light transmitting region 5 has an amplitude close to 1, and the light corresponding to the phase shift region 4 has an amplitude close to 0.
FIG. 4 is a vertical cross-sectional diagram of a photo-resist pattern 20, having patterns with dimensions larger than 0.25 .mu.m, formed using the phase shift mask of FIG. 1. FIG. 5 is a plan view illustrating the photoresist pattern 20 on the semiconductor substrate 10 in FIG. 4.
The photo-resist pattern 20 is formed by allowing light passing through the phase shift mask in FIG. 1 to reach a photoresist film applied on a semiconductor substrate 10. The exposed photoresist film is then developed with a developing solution. The portions of the photoresist film below the light transmitting regions 5 of the phase shift mask in FIG. 1 are exposed to light and changed such that the exposed portions can be dissolved by the developing solution. The portions of the photo-resist below the phase shift regions 4 are not exposed to the light, thus they are not able to be dissolved in the developing solution. Accordingly, when the semiconductor substrate 10 with the exposed photo-resist layer is put into the developing solution, which is typically an alkaline solution, the portion of the photoresist film below the light transmitting region 5 is dissolved and removed, thus forming the photoresist pattern 20 shown in FIG. 4. The photoresist pattern shown in FIGS. 4 and 5 is what one ideally wants to achieve. Unfortunately, when the feature dimensions become quite small, for instance, on the order of 0.25 .mu.m, problems can occur.
FIG. 6 is a flow chart showing the steps of a background art method of forming a photoresist pattern having dimensions larger than 0.25 .mu.m. First, in step 61, a wafer is provided into a processing track and a photoresist layer is coated on the wafer in step 62. The photoresist is cured by conducting a soft bake process in step 63. The wafer is then cooled down in step 64. Next, the wafer is put into an exposing apparatus, such as a stepper, and exposed to light using a half-tone phase shift mask in step 65. A post expose bake process is conducted with the resultant wafer in step 66. The wafer is then put into an alkaline developing solution to be developed, thereby removing the exposed portions thereof and forming a photoresist pattern in step 67. A hard bake process is then conducted with the wafer, thereby curing the photoresist pattern in step 68. Finally, the process for forming the photoresist pattern is completed by extracting the wafer from the track in step 69.
A chemical amplification photoresist is typically used in the process described above. The chemical amplification photoresist will often, include a novolak resin, a photo acid generator (PAG), a sensitizer, and a dissolution inhibitor. When such a photoresist is exposed to light, the PAG generates hydrogen ions (H.sup.+). Further, when the post exposure baking process is conducted at about 100.degree. C. for 30 minutes, the number of hydrogen ions rapidly increases and they diffuse into the photoresist, thus disconnecting links of the dissolution inhibitor. As the links of the dissolution inhibitor are disconnected, additional hydrogen ions are generated, and the generation of hydrogen ions is further amplified. This is why the photoresist is called a chemical amplification photoresist. When the links of the dissolution inhibitor are completely disconnected, the photoresist may be dissolved by an alkaline developing solution, such as tetramethyl amonium hydroxide (TMAH), thus forming the photoresist pattern.
When a chemical amplification photoresist is used in combination with a half-tone phase shift mask having extremely small dimensions, such as on the order of 0.25 .mu.m, and when opening patterns in the half-tone phase shift mask are formed adjacent each other photosensitization may occur, and some of the exposing light may penetrate the chrome light blocking portions. The light penetrating the light blocking portions partially exposes the underlying photoresist. This, in turn, will cause a portion of the photoresist which should not be removed to be partially etched during subsequent development steps. Thus, a side lobe in an undesired portion of the photoresist is opened.
FIG. 7A is a plan view of a photoresist film in which a side lobe 30a appeared when the photoresist film was exposed to light and developed using a half-tone phase shift mask as shown in FIG. 1 having very small feature dimensions. FIG. 7B is a vertical cross-sectional diagram of the photoresist pattern shown in FIG. 7A taken along section line VIIb--VIIb. In FIGS. 7A and 7B, a photoresist pattern 20, includes opening portions 30, and a side lobe 30a.
A clean room for fabricating a semiconductor device is typically provided with a processing track in which the process steps for forming the semiconductor device are actually conducted. An interface is provided for a person who works to manufacture the device. A photoresist patterning process is conducted in the track.
An atmosphere in the track is more filtered than that of the interface, thus lowering a density or concentration of particles and gases which are harmful to the semiconductor device manufacturing process. In the typical process for patterning a conventional chemical amplification photoresist, a density of alkaline gas such as NH.sub.3 in the track is controlled so that it remains under 1 ppb (parts per billion). This is usually achieved through a chemical filtering process. However, a density or concentration of the alkaline gas in the interface may be as high as approximately 20 to 50 ppb.
When a chemical amplification photoresist layer which has been exposed to patterning light using a mask is then exposed to a relatively high density alkaline gas such as NH.sub.3, for example above 50 ppb, the alkaline gas is neutralized by combining with the hydrogen ions generated in the photoresist at a surface thereof in response to the exposure light. The combination of hydrogen ions with the alkaline gas prevents the links of the dissolution inhibitor from being disconnected. Consequently, an exposed portion of the photoresist remains insoluble due to the continued presence of the dissolution inhibitor. Thus it is impossible to form the photoresist pattern. This is the reason why the density of the alkaline gas should remain under 1 ppb inside the processing track.